$-3st - 4su - 4s - 5 = 7t + 4$ Solve for $s$.
Explanation: Combine constant terms on the right. $-3st - 4su - 4s - {5} = 7t + {4}$ $-3st - 4su - 4s = 7t + {9}$ Notice that all the terms on the left-hand side of the equation have $s$ in them. $-3{s}t - 4{s}u - 4{s} = 7t + 9$ Factor out the $s$ ${s} \cdot \left( -3t - 4u - 4 \right) = 7t + 9$ Isolate the $s$ $s \cdot \left( -{3t - 4u - 4} \right) = 7t + 9$ $s = \dfrac{ 7t + 9 }{ -{3t - 4u - 4} }$ We can simplify this by multiplying the top and bottom by $-1$. $s= \dfrac{-7t - 9}{3t + 4u + 4}$